While on PCA morphine, the patient's respiratory rate, SpO2 and sedation score will be monitored every hour. Heart rate and blood pressure will be checked every 4 hours. NRS pain scores at rest, during knee movement, during sleep, cumulative PCA morphine doses, and number of PCA demands/goods delivered, and side effects (nausea, vomiting, dizziness) will be recorded every 4 hours. Patients will be assessed by pain team everyday.